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Posted to issues@spark.apache.org by "Eyal Farago (JIRA)" <ji...@apache.org> on 2018/08/13 16:05:00 UTC
[jira] [Commented] (SPARK-25103) CompletionIterator may delay GC of
completed resources
[ https://issues.apache.org/jira/browse/SPARK-25103?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16578519#comment-16578519 ]
Eyal Farago commented on SPARK-25103:
-------------------------------------
CC: [~cloud_fan], [~hvanhovell]
> CompletionIterator may delay GC of completed resources
> ------------------------------------------------------
>
> Key: SPARK-25103
> URL: https://issues.apache.org/jira/browse/SPARK-25103
> Project: Spark
> Issue Type: Bug
> Components: Spark Core
> Affects Versions: 2.0.1, 2.1.0, 2.2.0, 2.3.0
> Reporter: Eyal Farago
> Priority: Major
>
> while working on SPARK-22713 , I fund (and partially fixed) a scenario in which an iterator is already exhausted but still holds a reference to some resources that can be GCed at this point.
> However, these resources can not be GCed because of this reference.
> the specific fix applied in SPARK-22713 was to wrap the iterator with a CompletionIterator that cleans it when exhausted, thing is that it's quite easy to get this wrong by closing over local variables or _this_ reference in the cleanup function itself.
> I propose solving this by modifying CompletionIterator to discard references to the wrapped iterator and cleanup function once exhausted.
>
> * a dive into the code showed that most CompletionIterators are eventually used by
> {code:java}
> org.apache.spark.scheduler.ShuffleMapTask#runTask{code}
> which does:
> {code:java}
> writer.write(rdd.iterator(partition, context).asInstanceOf[Iterator[_ <: Product2[Any, Any]]]){code}
> looking at
> {code:java}
> org.apache.spark.shuffle.ShuffleWriter#write{code}
> implementations, it seems all of them first exhaust the iterator and then perform some kind of post-processing: i.e. merging spills, sorting, writing partitions files and then concatenating them into a single file... bottom line the Iterator may actually be 'sitting' for some time after being exhausted.
>
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